Abstract

We have studied the relationship between the solar-wind speed \([V]\) and the coronal magnetic-field properties (a flux-expansion factor [\(f\)] and photospheric magnetic-field strength [\(B_{\mathrm{S}}\)]) at all latitudes using data of interplanetary scintillation and solar magnetic field obtained for 24 years from 1986 to 2009. Using a cross-correlation analyses, we verified that \(V\) is inversely proportional to \(f\) and found that \(V\) tends to increase with \(B_{\mathrm{S}}\) if \(f\) is the same. As a consequence, we find that \(V\) has an extremely good linear correlation with \(B_{\mathrm{S}}/f\). However, this linear relation of \(V\) and \(B_{\mathrm{S}}/f\) cannot be used for predicting the solar-wind velocity without information on the solar-wind mass flux. We discuss why the inverse relation between \(V\) and \(f\) has been successfully used for solar-wind velocity prediction, even though it does not explicitly include the mass flux and magnetic-field strength, which are important physical parameters for solar-wind acceleration.

Notes

Acknowledgements

The IPS observations were carried out under the solar-wind program of the Solar-Terrestrial Environment Laboratory (STEL) of Nagoya University, and were partially supported by the IUGONET Project of MEXT, Japan. The authors would like to express their thanks for the use of the NSO/Kitt Peak magnetogram data. They are also indebted to the NASA/GSFC’s Space Physics Data Facility’s OMNIweb service, and OMNI data. This work was carried out by the joint research program of the STEL, Nagoya University, and was also partially supported by the JSPS Grant-in-Aid for Scientific Research A (25257079).